An interactive tool produced by the group Climate Central illustrates how rising temperatures and reduced snowpack in the western U.S. have corresponded with an increase in wildfires in recent decades. Based on federal wildfire data from 1970 to 2012, the graphic shows how large fires in some western states — including Arizona, Colorado, and Idaho — have doubled or even tripled in four decades, a period when the average spring and summer temperatures in 11 states increased by more than 1.5 degrees F. According to the Climate Central analysis, Arizona has experienced the highest average increase in spring temperatures, about 1 degree F, which has likely been a key factor in the steep increase in fires covering more than 1,000 acres. Another key factor has been the decrease in mountain snowpack, as measured by the amount of snow and water on the ground on April 1. During several seasons, unusually low amounts of spring snow caused extended droughts that helped drive more big fires. In Nevada, where snowpack has been declining since the early 1980s, a record low snowpack in 2012 helped trigger the worst wildfires in more than a decade. Colorado also experienced record spring and summer temperatures in 2012, which were followed by extensive wildfires.